Torque limiting bolts are used in many applications where it is required to provide a certain tension in the shank upon the tightening. A well known example is in the bolting of steel plates or beams in the construction industry. In such applications, the installation of a bolt is commonly made with access to one side only of the parts to be joined and the bolt and nut are tightened by a power wrench. In this application, the bolt is provided with a retaining head at one end of the shank and a tightening head at the other end. The threaded portion of the shank adjacent the tightening head receives a conventional nut, such as a hex head nut. The power wrench, typically a nonimpacting electric wrench, has a chuck with an inner socket for receiving the tightening head and an outer socket which receives the nut. The sockets are counter-rotated by the wrench for tightening the nut onto the bolt. The bolt is provided with a shear neck or twist-off neck to provide tension control in the tightening of the bolt. When the torque reaches a predetermined value the tightening head is twisted off the bolt shank.
The use of a power wrench for installation of such torque limiting bolts is highly efficient and lends itself to high productivity due to the use of a power wrench requiring only one hand for operation to install the bolt. This is highly advantageous in hard-to-reach installations such as those frequently encountered by steel workers. In practical application of such a fastener installation system, a difficulty has been encountered which tends to detract from the advantages gained by the power wrench installation. The difficulty is that the tightening head of the bolt tends to bind or jam in the socket and after it is twisted off and it is not ejected from the socket. The conventional bolt is provided with a twelve point head of standard configuration. This head mates with the inner socket in a loose fit to facilitate easy mating engagement. The size of the head will vary from its nominal dimension by the allowable manufacturing tolerances. In order to accommodate the largest head which is within tolerance, the inner socket must be somewhat larger to provide a proper fit. This relationship results in an undesirably loose fit in the case of the smallest head which is within manufacturing tolerance. It is believed that this extreme case of looseness allows relative rotation between the head and the socket which results in a camming action as the nut is tightened. The camming action between the facets of the head and the socket results in a binding or jamming of the head in the socket so that it cannot be ejected with the normal ejection force of the tool and the socket has to be cleared of the head by manual force.
A general object of this invention is to provide an improved torque limiting bolt which coacts with a driving socket in such a manner as to overcome certain disadvantages of the prior art.